I don't really blame players who use the game as their dating site - there are way too many stories about game relationships turning into RL ones. No, it's not common, it's insanely rare. Yet desperate people do desperate things. Ironically if they were not pushing so hard maybe they actually could achieve what they are looking for.
It's important to understand that the game isn't a single player game. It's a multiplayer game. As such the RL is bound to more or less affect the in-game interactions. There are also many steps between the in-game and RL. Some players consider RL anything that is outside the game - for example chat/facebook/skype etc. - while others may think that only face to face interactions are considered RL.
Now for the proving your RL sex thing... I don't understand the issue. If you admit you are male/female then where is the harm to say few words over the voice chat? (There might be unrelated issues - for example my pronunciation sux so I am in no hurry to talk over voice chats ^^ - but in my opinion it's better to be honest about the cause of your behavior). It may be just a game, but we need to consider people's feelings. Trust is a really nice thing, but the uncertainty could potentially harm the relationship - people are naturally and rightfully afraid that others on the Internet could lie to them. Imagine that you would develop feelings for a person and then found out that they are not what they said or what you thought they were - be it sex, age or whatever - that could be devastating and so it's fine to limit the risk of this. I doubt that anyone would be happy to find out that their supposedly middle-aged male eRP partner was an underage girl iRL etc.
So yeah, we have to be careful when dealing with people's feelings and the more honest you can be (and the less doubt there is about what you say) the better.





